1. Field of the Invention
The present invention relates to a method and installation for dephosphorizing pig manure by separating the manure into a solid and liquid fraction and removing phosphate from the liquid fraction.
2. Background Art
The consequences for the environment of over-manuring, such as phosphate saturation, acid rain, ground and drinking-water pollution, are known. The manure surplus problem has resulted in different proposals for the solution thereof. It has not however proved possible up to the present to solve the problem in an economically acceptable manner.
Pig manure, but also manure from poultry, contains phosphor in the form of phytate. The phytate comes from different components of the animal feed. Phytate is the salt of phytinic acid, dihydrogen phosphate, and consists of a sugar molecule with six phosphate molecules. Single-stomach animals such as pigs and chickens are not capable of absorbing sufficient phosphate from the feed. The greatest part of the phytate therefore finds its way, substantially undigested, into the manure.
Because animals such as pigs and poultry need phosphor, inter alia for a good bone structure, and they are themselves unable to absorb sufficient phosphate from their feed, extra phosphor is often added to the feed. The result is a further phosphate impact on the environment.
In order to avoid the addition of phosphor it has recently been proposed to add the enzyme phytase to the feed. Phytase degrades the phytate and makes phosphate available to the animals.
The whole supply of phosphate is still not absorbed by the animals from their feed. On the one hand not all the phytate is broken down by phytase, normally only about 75%, and on the other not all the degraded phosphate is also actually absorbed by the animals. A residual quantity of phosphate therefore remains in the manure. In order to prevent a phosphate saturation of the environment it is therefore important to dephosphorize manure.
European application 0 287 152 describes a method for producing amino acids from manure. As an additional step in the described process, phosphate is removed from the manure by separating the latter into a solid and a liquid fraction, whereafter the liquid fraction is concentrated by means of ultrafiltration. The remaining concentrate then contains, among other things, phosphate.